Optical communication systems are a substantial and fast-growing constituent of communication networks. Optical systems are employed in telecommunications systems, cable television systems, and local area networks (LANs). A laser diode is employed for transmitting an optical signal over an optical fiber in such an optical communication system. The laser diode is driven with a constant current supplied by a laser driver. A laser driver for use in an optical signal transmitter in an optical communication system selectively supplies laser-driving current responsive to a digital data signal received. In certain applications, it is desirable to resynchronize the data being transmitted with a clock, for example, when high speed data rates compromise signal integrity. Therefore, some conventional optical network laser drivers operate both in a latched mode and a transparent mode. The latched mode synchronizes the data signal with a clock signal prior to transmission of the signal to the laser diode to reduce jitter associated with the system. The transparent mode transmits the data signal directly to the laser diode.
FIG. 1 illustrates a conventional optical driver 10 for driving an optical network laser light emitting diode (LED). The optical driver 10 includes a latch device 16 (e.g., master-slave latch) and a pair of buffers 12. The latch device 16 receives a data signal (DATA) and a clock signal (CLOCK). The latch device 16 synchronizes the data signal to the clock signal and provides the synchronized data to a first input of a multiplexer 14. The buffers 12 also receive the data signal and provide a buffered version of the data signal to a second input of the multiplexer 14. The buffers 12 can be simple current mode logic (CML) amplifier stages.
The output of the multiplexer 14 is provided to an output driver 18. The output driver 18 drives a laser light emitting diode (LED) according to the data sequence and data rate of the data signal. A select line (SEL) is coupled to the multiplexer 14, the buffers 12 and the latch device 16. The logic state of the select line determines whether the synchronized version of the data signal via the latch device 16, or the buffered version of the data signal via the buffers 12 is to be transmitted to the output driver 18.
The power required at the output of the driver 18 to the laser LED is substantially higher than the power of the data into the optical driver 10. Therefore in designing the optical driver 10, each successive stage in both the latch device 16 and the unlatched buffers 12 operate at gradually increased power levels to optimize propagation delay. Consequently, the multiplexer 14 operates at high power levels, required by the final output driver 18. The power consumption of this multiplexer 14 can be responsible for a significant fraction of the total chip power consumption. The power consumed by the multiplexer 14 must not only be sufficient to supply the base circuit in the final output devices, but also large enough to overcome the parasitic capacitance associated with devices in the non-selected path of the multiplexer 14. Also, the buffers 12 and the latch device 16 contribute to the overall real-estate and power consumption even when they are not part of the selected data path.